Is it just when going over vigorous bumps? Or crater sized potholes? Has the issue become gradual rather than just instant? If so then;

Id check the rear axel bushes, could possibly be this, either that or the suspension bushes. If its not them then you may have to invest in some new shock absorbers (roughly £20, slightly more expensive than bushes £few).

If its randomly just appeared and is loud then id check the rear anti-roll bar and rear suspension as something may be loosened, either way get it checked over asap as if its the latter of the suggestions then it could certainly be very dangerous.